The genetic diseases you mentioned are significant mutations leading into strong dysfunctionalities in how the brain is supposed to function. Leading it not to function correctly. This is far from evolutionary path leading one population of people into some sort of mental retardation, especially when evolutionary selection would be against it. In addition most of the mentioned diseases are recessive which is the only reason they have survived, as any offspring with the disease won't survive to pass the genes on.

Autism is a different case because it's not necessarily genetic (although I'm not sure if there are also genetic cases). In fact autism cases have exploded in recent history, so environmental factors are likely a contributor. One such theory is the vaccines containing mercury which is a strong neuro-toxin decaying neuron connections which would result into the observed behavior. I'm not going into that discussion however.

I'm being unspecific about intelligence because it's inherently a very ambiguous concept, which is one of the fallacies of the IQ testing and the arguments given by the racists like in this thread. I can see short comings and non-objectiveness in almost every IQ test I have seen. And that without even taking into account the fact that they're "adjusted" periodically to reflect the norm which obviously means they're not perfectly objective to begin with.

I was referring to 18th century Germany and for example theories created by Johann Friedrich Blumenbach.

http://en.wikipedia.org/wiki/Blumenbach